Synchronous duplicating with comparison, multiple redundancy with majority selection, and parity checking, as well as time supervision in the form of "watch dog" and microprogrammed pointer control, are known principles for supervising a data processing system. Even if it is only partially used, duplication is very expensive in many cases. Parity checking requires a rather considerable increase in the number of components used. Pointer control and parity generation mean increased access times relative to the data processing. Time supervision results in tardy error detection.
The state of the art is dealt with, for example, in "12th Annual International Symposium on Fault-Tolerant Computing" (ISSN number 0731-3071), Session 6B On-Line Monitoring, pages 237-256.